Electrical submersible pumps (ESP) are often employed to pump well fluid from wells. A typical ESP includes a rotary pump driven by an electrical motor. Normally, the ESP is suspended in the well on a string of production tubing. A drive shaft assembly extends from the motor through a seal section and into the pump for driving the pump. The motor and at least part of the seal section are filled with a dielectric motor lubricant.
The seal section has a main seal that seals around the shaft, sealing well fluid on the exterior from the motor lubricant. Normally, the main seal is a mechanical face seal having a rotating member that rotates against a stationary member. Mechanical face seals of this type are durable and work well, but they weep a small amount depending on the matrix of the well fluids, such as the presence of solids, sand or scale, which can degrade the sealing interface over time. The entry of well fluid past the main seal allows well fluid to come into contact with the motor lubricant. Seal sections may also have a secondary mechanical face seal at the end facing the motor.
One type of seal section has a labyrinth arrangement that creates a serpentine flow path for fluid to flow from the pump end to the motor end. U.S. Pat. No. 8,845,308 discloses a labyrinth type seal section particularly for use in horizontal sections of wells. A number of discs are disposed within the seal section housing. Each disc has a communication port extending through it. The discs are oriented with the communication ports at different angular locations. When the ESP is operated horizontally, some of the communication ports will be at different elevations than others. Water within the encroaching well fluid tends to accumulate in the lower portions of the housing between the discs. The higher and lower communication ports inhibits the migration of water horizontally from the pump end to the motor end of the seal section.
Another type of labyrinth seal section is intended particularly for vertical orientations of the ESP and comprises at least one downward extending flow tube with a lower outlet in the chamber and at least one upward extending flow tube with an upper outlet in the chamber. Any well fluid leakage has to flow down the downward extending flow tube, then back up the upward. The well fluid and the motor lubricant may have a contacting interface, such that hydrostatic pressure from the well bore fluid is applied to the motor lubricant to equalize with the hydrostatic pressure of the well fluid on the exterior of the ESP.
Some types of seal section also have pressure equalizing components, such as a flexible bag or bellows. The bag or bellows has motor lubricant on one side and well fluid on another side. Seal sections with pressure equalizing components may also have some type of labyrinth arrangement. Also, a separate pressure equalizing unit may be attached to the end of the motor opposite the seal section. Seal sections may also include a thrust bearing unit for absorbing thrust on the drive shaft.
The various types of seal sections typically employ a main seal of a type that can weep in the event the sealing surface begins to degrade, as mentioned above. It is known to add a second mechanical face seal to the seal section for redundancy. However, the seal section would normally need extensive re-design to locate the second mechanical face seal. It is also known to add an additional seal section in tandem, the additional seal section having a second mechanical face seal. A second seal section in tandem adds to the cost of the ESP significantly and may increase the horsepower requirements of the motor.